Travelling wave oscillators



July 3, 1956 M. ETTENBERG TRAVELLING WAVE OSCILLATORS Filed June 14, 1952 n NT m WT m mE M S l m RY United States Patent llice 2,753,481 TRAVELLING WAVE OSCIIVLLATORS Morris Ettenberg, New York, N.. Y.,V assigner to Sperry Rand. Corporation,i a corporation. of Delaware Application: June I4, `1:952, SerialN'o. 293,501V 8 Claims.. (,Cl. 315-16.)

This invention relates to improvements in high frequency oscillators of the travelling wave. type, and one of its principal` objects is` to. provide travelling, wave oscillators capable of being tuned throughout a substantial frequency band by variation of a control voltage applied thereto.

Another object is to provide electrically tunable oscillaters which operate with` relatively high overall eiiiciency and. provide substantially constant output over the tuning range.

A further object is to provide, oscillators of the described type which are of simple construction, requiring a minimum of external apparatus such as waveguides and circuit elements.

The invention will be described with. reference to the accompanying drawing, wherein:

Fig. l depicts one embodiment of the invention showing a special vacuum tube,` which constitutes a major part of the system, in longitudinal section,

Fig. 2 is a graph showingd the relative wavepropagation velocities as functions of frequency on two conductive helix elements in the system of Fig. l, and

Fig. 3- is a longitudinal section of a modification of the tube shown in Fig. l.

Usual travelling wave amplifiers show a tendency to oscillate at one or more frequencies where the effective length of the helix or equivalent wave propagating structure is such that a wave reflected back from the output end will arrive at the input end in such phase as to reinforce the input. To prevent oscillation, the helix may be made of lossy material throughout, or may be provided with substantially lumped attenuation at an intermediate point.

Although a travelling wave tube may be made to oscil late by simply omitting the attenuation, `its utility as an oscillator is limited because there is no simple way to control the frequency. Also, it may oscillate at two or more frequencies simultaneously, or change erratically from one frequency to another.

According to the present invention, a travelling wave tube is made with two independent helices arranged to be traversed in succession by the same electron beam. The first helix is designed with low loss to act as an oscillator, and the second helix is designed as an amplifier, with its input supplied solely by the modulation produced on the electron beam by the oscillator section.

The travelling wave tube shown in Fig. l includes an electron gun 1 and a collector electrode 3 at the respective ends of a tubular vacuum envelope 5, which may be made of glass. The gun 1 is of conventional design, and includes a cathode assembly 7 and an accelerator electrode 9 arranged to direct a beam of electrons along the axis of the tube to collector 3. An external magnet, not shown, may be provided to produce al magnetic lield directed longitudinally of the tube for maintaining the beam in focus.

A conductive helix 11 is supported within the envelope and is terminated at its end nearer the collector elec- Patented lIuly 3, 195,6.

2: trodeby an arrangement for couplingit to anoutput wave guide 13. In the ing the helix conductor with an extension 15 connected to a short conductive collar 1.7. The wave guide 13 has apertures in its broad walls to accommodate the envelope 5, and is` placed across` the member 15 as shown. The end 14 of the guide 13. extends beyond thettube envelope as shown, and is provided with a short circuitingl plug 16. The position of the plug 16 is adjusted to. provide impedance matching between the guide 1.3 and the coupling member 15. A conductive flange 19 connected to the guide 13- surrounds. the envelope 5 adjacent the collar 17.

The helix 11 is designed. in known manner to cooperate with the beam produced` by the gun 1 to function as a travelling wave amplier over the frequency band throughout which the system is. tooperate. As in usual practice, the helix 11 is made of lossy material;` or provided with localized attenuation to` prevent self-oscillation. The in-` put end of the helix 11 is not provided with any external radior frequency coupling means. The end 21 of the helix conductor may be brought out through a seal in the envelope 5 for maintaining the helix at a suitable D.C. potential, such as ground.

Another conductive helix 23 is` disposed between the input end of the helix 11 and the electron gun 1. The helix 23 is of smaller diameter than the helix 11, and is supported within the envelope 5 inside a sleeve 25 of lowloss dielectric material suchV as fused quartz. The sleeve 25V lits` snugly within the envelope 5, and is maintained in position by dimples 28 formed in the envelope.

The helix 23. is made of low-loss material such as copper, and is not provided with any attenuator means. Both its endsV are reilectively terminated by impedance discontinuities. In the minates in an open circuit. The left hand end of the helix conductor extends through the wall of the en velope as at 27, for connection to a D.C. supply and/or control signal source. This provides an R. F. mismatch which may approximate a short circuit or some finite impedance, depending upon the length of the tere minal 27 and how it is by-passed, as by a capacitor 30.

A power supply source, such as a battery 29, is connected to provide suitable operating potentials for the various tube elements. This may be effected by connecting the source 29 to place the cathode assembly 7 at a high engative potential with respect to ground, and grounding the accelerator 9, collector 3, and helix 11. The os cillator helix 23 may be connected to the source 29 by way of an adjustable voltage divider 31 as shown, in such manner that the potential of the helix 23 may be varied throughout a substantial range.

Referring to Fig. 2, the velocity of wave propagation, as a function of frequency, on the helix 11 is represented by the solid line 33. The velocity is approximately constant at a value vo throughout a wide range of frequency above the frequency f1. The velocity vu is approximately equal to the axial component of velocity of a wave travelling on the helix conductor with the velocity of light; its value depends upon the helix dimensions, being smaller as the helix pitch is made finer and the helix diameter is made larger. The frequency f1 also depends on the helix dimensions, being generally lower as the helix pitch is made coarser and larger in diameter.

Below the frequency fr, the velocity increases with decreasing frequency, at a rate which also increases with decreasing frequency. This change of velocity with frequency is called dispersion; the helix 11 is dispersive below the frequency f1.

The dash line curve 35 in Fig. 2 represents thevelocity vs. frequency characteristic of the helix` 23. The helix 23, being smaller than the helix 11, begins t0 .GXhibit disdispersion at a higher frequency, fr. The velocity vo at present example this .is effected by form-` present example, the right hand end ter--y frequencies above f2 may be less than v0, as shown, or more, depending upon the design of the helix 23. In either case, the helix 23 is dispersive throughout the range f1 to f2, and the helix 11 is substantially non-dispersive or has relatively low dispersion throughout this same frequency range.

In the operation of the system, the electron beam produced by the cathode assembly 7 is accelerated to a velocity which depends upon the potential difference between the accelerator electrode 9 and the cathode. If the potential of the helix 23 happens to be the same as that of the accelerator 9, the beam enters and passes through the helix 23 without change in its average velocity. If the helix 23 is at a different potential, the beam is accelerated or decelerated, as the case may be, in going from the accelerator to the helix 23. Its velocity in the helix 23 corresponds to the potential difference between the cathode and the helix 23. Thus, the beam velocity in the helix 23 may be varied, independently of its value at the accelerator by adjustment of the voltage divider 31.

Similarly, the velocity of the beam in the helix 11 corresponds to the potential difference between this helix and the cathode, and is substantially independent of its velocity elsewhere. In the present example, it will be the same as that at the accelerator electrode because they are both grounded, but this is not essential. The potential difference between the helix 11 and the cathode is adjusted, as by adjustment of the source 29, to make the beam velocity in the helix 11 substantially equal to the wave veloicty vo.

The beam velocity in the helix 23 is adjusted by means of voltage divider 31 to a value u within the dispersive range corresponding to sorne frequency f between f1 and f2 (Fig. 2). Then any random disturbance having a component of frequency f will start a wave of this frequency travelling along the helix at the same velocity as the electron beam. This wave will exchange energy with the beam, modulating the beam and converting some of its kinetic energy into radio frequency energy which adds to the original wave, producing amplification.

The amplified wave on the helix is reflected by the impedance discontinuity at the right hand end of the helix, and travels back to the left hand end, where it is reflected again. The twice-reflected wave is amplified again, and reected againto reappear at the left hand end of the helix. Thus, providing the electrical length of the helix is such that the wave continues to reinforce itself, continuous oscillation will occur at the frequency f.

The amplitude of the resulting modulation on the electron beam will depend on various factors, including the length of the helix and its relationship to the Wavelength. As the beam velocity is varied the frequency at which the wave velocity corresponds to the beam velocity will vary. The electrical length of the helix depends on the frequency, too, so the effective feedback will vary, being a maximum when the total phase shift around the path is exactly 21|- radians, and decreasing as the frequency departs from that value. The result is that the modulation on the beam varies with frequency like a resonance curve.

The modulated beam enters the helix 11 and induces on it a wave of frequency f, like the wave on the helix 23. This vwave is amplified as in an ordinary travelling wave amplifier, by interaction with the electron beam. However, the helix 11 is made long enough to produce so much gain that the beam becomes substantially saturated whenever the amplitude of the modulation produced by the helix 23 is more than a relatively small value. Thus the wave Yappearing at the output end of the helix 11 will be of approximately constant amplitude throughout a substantial range of variation of the frequency. The output wave is conducted by the guide 13 to a utilization device, not shown. The electron stream is collected by the electrode 3, completing the circuit to the source 29.

Although the frequency-varying device may be simply a voltage divider as shown, itwill be apparent that any othermeans for varying the potential of the helix 23 may be substituted or added. For example, the system may be frequency modulated by including a source of modulation signal in the connection to the terminal 27.

The range fr to f2 of useful dispersion of the helix 23 may in general be much greater than the range over which the electrical length of the helix is such as to permit continuous oscillation in a single mode. As the control voltage is increased or decreased beyond the limits of oscillation in one mode, the beam velocity will approach correspondence with the wave velocity at another frequency where the phase relations again produce oscillation. Thus the system may be made to operate over two or more separate frequency ranges or modes like those observed in the operation of reflex klystron oscillators.

Although it is considered preferable at present to make the amplifier helix 11 substantially non-dispersive throughout the operating frequency range as indicated in Fig. 2, this helix may also be made dispersive like the oscillator helix 23. In this case, the terminal 21 may be connected to the voltage divider 31 or other suitable source so that the electron velocity will always correspond substantially to the wave velocity. Alternatively, both helices 11 and 23 may be grounded, and the potential of the cathode as sembly varied with respect to ground for controlling the frequency.

With the structure of Fig. l `the electron beam must be made small enough to go through the helix 23 and consequently may not be large enough for optimum interaction with the helix 11. Also, when high frequencies are involved, the helix 23 may need to be made so small that the power handling capability of the system is limited by the necessity for making the beam correspondingly small.

When power output and eiciency are important considerations, particularly at high frequencies, the structure of Fig. 3 may be preferable to that of Fig. l. In this case, the oscillator helix 23 is supported on a case 37 of low-loss insulating material and the electron beam is tubular so as to tlow outside the helix 23 and inside the helix 11.

The required hollow beam is produced by an electron gun shaped substantially like a figure of revolution of a section of an ordinary electron gun, about an axis outside the section. As shown in Fig. 3, the electron gun comprises an annular cathode 39 and focussing electrode 41, and the accelerator electrode includes inner and outer conductive members 43 and 45 defining a circular slit 47 which conforms in cross section to the desired electron beam.

In the illustrated design, the outer part 4S of the accelerator electrode has a skirt which acts as part of the vacuum enclosure, surrounding the cathode and focussing electrodes. The inner part 43 is supported on a hollow stem 49 secured to the end wall 51. The helix core 37 is supported on a rod 53 which extends into the stem 49. The D.-C. or control voltage terminal 27' for the helix 23 extends through the stem 49 and a hole in the rod 53, and goes out radially of the core 37 to the end of the helix. The other end of the helix is terminated in an open circuit. The remainder of the structure, including the output coupling arrangement 13, 15, 17, and the collector 3, may be the same as in Fig. l.

The adjustment and operation of the system of Fig. 3 is substantially the same as that of Fig. l, except for the differences in performance resulting from the use of the tubular beam.

Since many changes could be made in the above construction and many apparently widely different embodiments of this invention could be made without departing from the scope thereof, it is intended that all matter contained in the above description or shown in the accompanying drawings shall be interpreted as illustrative and not in a limiting sense.

What is claimed is:

l. A travelling wave oscillator tube including two conductive helices disposed end to end on a common axis,

means terminating the first of said helices in extreme impedance discontinuities at its ends, attenuator means providing substantial high frequency loss along at least a portion of the second of said helices, output coupling means at the end of said second helix further from said first helix, and means for producing and projecting a beam of electrons along said axis past said first helix and said second helix in succession, the first of said helices being designed to be substantially dispersive with the second of said helices being designed to have a minimum of dispersion throughout a predetermined wide operating frequency range.

2. A travelling wave oscillator tube including two conductive helices disposed end to end on a common axis, the first of said helices being of relatively louI loss and terminated in extreme impedance mismatched at its ends, one of said mismatches comprising an electrically open circuit, the second of said helices being of relatively high loss, means for coupling the end of said second helix remote from said first helix to an output wave guide, and an electron gun located near the end of said first helix further from said second helix and positioned to project a beam of electrons along said rst helix and said second helix in succession; said first helix being designed to be substantially dispersive with said second helix being designed to have a minimum of dispersion throughout the operating frequency range of `the device.

3. A travelling wave oscillator tube including a substantially dispersive slow wave propagating structure having a velocity of propagation which is an inverse funetion of frequency over a predetermined wide frequency range, means refiectively terminating said structure at its ends, one of said ends being terminated by an electrically open circuit, an electron gun located near one of said ends and positioned to project a beam of electrons along said structure in energy exchanging relationship with waves travelling thereon, whereby said beam and said structure cooperate to produce high frequency wave energy and modulate said beam therewith, and means for varying the average velocity of said beam to vary the frequency of said oscillations; a second slow Wave propagating structure disposed end to end with said first structure along the path of said beam, said second structure having a substantially constant velocity of propagation over said predetermined wide frequency range, said second structure including attenuator means to prevent self-excited oscillation thereof, and output coupling means at the end of said second structure remote from said first structure.

4. An electrically tunable high frequency oscillator, including a travelling wave tube wtih an electron gun adapted to project a beam of electrons along a longitudinal axis, two conductive helices disposed end to end along said axis, the first of said helices being substantially dispersing over a predetermined frequency range and of relatively low loss material and adapted for self oscillation over said frequency range by virtue of wave reflections at its ends, the second of said helices being of substantially larger diameter' than the first for providing minimum dispersion over said frequency range and being provided with attenuation to prevent self-oscillation, a direct current source connected to said electron gun and to said second helix, and adjusted to make the velocity of said beam in said second helix correspond to the velocity of wave propagation on said second helix, means for supplying a direct current potential to said first helix, and means for varying said potential to vary the veloicty of said beam through said first helix to vary the frequency of oscillation of said first helix.

5. An electrically tunable high frequency oscillator, including two conductive helices a common axis, the first of said helices being of relatively low loss material and provided with extreme electrical discontinuities at its ends, one of said discontinuities conidisposed end to end along i' prising an electrically open circuit, the second of said helices being of substantially larger diameter than the first and being provided with attenuation to prevent selfoscillation, said first of said helices being adapted to be substantially dispersive with said second of said helices having a minimum of dispersion over a predetermined wide frequency range, an electron gun adjacent the end of said first helix remote from said second helix, said gun comprising annular cathode and focussing elements to produce a tubular beam having an inner diameter larger than said first helix and an outer diameter smaller than said Second helix, said gun being positioned to project said beam over the outside of said first: helix and through the inside of said second helix, and output coupling means at the end of said second helix remote from said first helix.

6. A travelling wave tube oscillator, comprising first and second electromagnetic wave conducting helices liaving aligned axes, means including a cathode for producing and directing an electron beam along said axes past said first and second helices in succession, said first helix being substantially dispersive throughout a relatively wide range of high frequencies and having its ends terminated in extreme impedance discontinuities for supporting standing waves on said helix at frequencies within said range, one of said discontinuities comprisng an electrically open circuit, said second helix being substantially non-dispersive over said frequency range and of substantially greater axial length than said first helix, means coupled to said first helix for supplying a direct current potential thereto, and means for varying said potential for selectively producing oscillations along said first helix at frequencies within said frequency range to thereby modulate said electron beam, said second helix comprising high gain amplification means for amplifying travelling waves induced thereon.

7. A travelling wave tube oscillator as forth in claim 6, wherein said first helix is smaller di vneter and pitch than said second helix and is substa tially shorter than said second helix.

8. A travelling wave tube oscillator, comprising iirst and second electromagnetic wave conduct-infr helices having aligned axes, means including a cathode for producing and directing an electron beam alc-ng said axes past said tirst and second helices in succession, the first of said helices being substantially dispersive throughout a relatively wide range of high frequencies, the end of said first helix adjacent said cathode being short-circuited to ground and the opposite end thereof being substantially open circuited for terminating the ends of said first helix with extreme impedance discontinuities, means coupled to said first helix for supplying a direct current potential thereto, and means for varying said potential for selectively producing oscillations along said first helix at frequencics within said frequency range to thereby modulate said electron beam, said second helix. comprising high gain amplification means for amplifying travelling waves induced thereon.

References Cited in the file of this patent UNITED STATES PATENTS 2,584,308 Tiley Feb. 5, 1952 2,603,773 Field July l5, 1952 2,645,737 Field July 14, 1953 FOREIGN PATENTS 978,576 France Nov. 29, 1950 668,334 Great Britain s Mar. 12, 1952 671,296 Great Britain Apr. 30, 1952 OTHER REFERENCES An Internal Feedback Traveling-Ware-Tube Oscillator, Jones, Proc. of I. R. E., April 1952, pp. 478-482 

